Effective communication between patients and providers is an important component of chronic illness care. Electronic messaging shows promise as a tool to support communication between patients with chronic disease and their providers. Little is known, however, about the impact of using the Web in the clinical care of patients with chronic disease. In the present study, we aim to better understand the role of electronic messaging in the care of patients with chronic medical conditions. We propose to conduct a retrospective analysis of a large and uniform patient secure messaging application at Group Health Cooperative (GHC). We will study patients with one or more of the following conditions: depression, congestive heart failure and diabetes. For each of these populations we will first describe the characteristics of patients who choose to use SM with their healthcare provider. Second, for patients with diabetes, we will evaluate the potential association of SM use with safer and more effective medical care. The results of the above aims will help inform the development and evaluation of future online health services for patients with chronic medical conditions.